Permanent magnet



June 19, 1956 M. HEKELAAR 2,751,525

PERMANENT MAGNET Filed July 6, 1954 INVENT MAARTEN H LAAR BY W MW PERMANENT MAGNET Maarten Hekelaar, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application July 6, 1954, Serial No. 441,549

tllaims priority, application Netherlands July 24, 1953 4 Claims. (Cl. 317-201) The invention relates to a bored permanent magnet consisting of magnetic oxidic material having a high retentivity and a high coercivity of the formula MFemOrs, where M represents one or more of the metals Ba, Sr or Pb, a shaft being secured in the bore provided in the permanent magnet by means of a cement.

In a known manner of securing a permanent magnet to a shaft the space between the magnet and the shaft is completely filled by the cement. If required, the cement may be provided with a filler to enable undue hollow spaces of the bore to be filled.

It has been found that the known attachment for a permanent magnetic material of the kind to which the invention relates does not fulfill the requirements which this attachment must satisfy. This is due to the fact that the above-mentioned permanent magnetic material is porous with the result that during the securing process a large part of the cement is absorbed by the permanent magnetic material. Consequently the magnets will work loose from the shaft in course of time. In addition the cement will be able via the porous magnetic material to absorb substances from its surroundings, for example, moisture or oil, with the result that the cementing properties will be detrimentally affected. These detrimental effects will be more important as the diameter of the shaft is smaller, since when a load is imposed the stresses produced in the attachment of the shaft to the magnet will be greatest at the smallest diameter, that is at the point at which the cement is secured to the shaft.

According to the invention, in order to obviate the above difficulties, a sleeve is arranged in the bore so as to surround the shaft, which sleeve is made of non-porous material and is secured to the permanent magnet and to the shaft with the aid of a cement.

Thus, the advantage is obtained that the cement provided between the shaft and the sleeve is protected from the action of detrimental substances by the sleeve made of non-porous material so that in this event the cementing properties are completely maintained. As has been explained hereinbefore, this attachment is very important, since in the loaded condition the greatest stresses are produced at this point. It is true that the cement layer provided between the sleeve and the permanent magnet according to the invention are not protected from detrimental substances, however, this is not inconvenient since at this point the attachment is effected at a much larger diameter with the result that in the loaded condition the stresses produced at this point are much smaller than in the cement layer provided between the shaft and the sleeve. For the same reason the absorption of the cement by the porous magnetic material which may occur does not inconveniently aifect the attachment of the magnet to the sleeve.

The diameter of the bore formed in the permanent magnet may be made such that any stresses which may be produced in the cement layer provided between the shaft and the magnet do not exceed the maximum stress which nited States Patent Patenteddune 19, 1956 the cement is able to absorb after the absorption of substances detrimentally affecting the cementing properties and/or after the absorption of the cement by the magnetic material.

The sleeve may be made of a non-porous ceramic material, since this material is very cheap and a cement readily adheres to it. Preferably the cement consists of a synthetic resin, for example the one known under the name of Araldite. In a preferred embodiment the difference in diameter between the magnet and the sleeve and between the sleeve and the shaft is about 0.3 mm. while the diameter of the bore formed in the magnet is about two times the diameter of the shaft.

The invention will now be described with reference to the accompanying drawing in which a sectional view of one embodiment of a permanent magnet secured to a shaft is shown diagrammatically. A permanent magnet 1 of the kind to which the invention relates is provided with a bore 2. A sleeve 3 made of non-porous ceramic material is secured in the bore 2 by means of a layer 4 of ethoxyline resin arranged between the sleeve 3 and the permanent magnet 1. A shaft 5 preferably made of metal is secured in the sleeve 3 also by means of a layer 6 of ethoxyline resin. In one embodiment in which the attachment of a permanent magnet made of a material known under the name of Ferroxdure to a metal shaft of a bicycle dynamo is constituted in accordance with the invention by a sleeve made of ceramic material sintered to compactness and by a cement known under the name of Araldite, it was found that this attachment collapsed only after a load of 1 /2 tons was imposed on it.

The material known under the name of Ferroxdure is fully described in Philips Technical Review, vol. 13, No. 7, pages 194-208. It is an oxidic material composed of hexagonal crystals having a composition MFeizOis in which M is one or more of the metals barium, strontium or lead and in which a portion of one of those latter metals may be replaced by calcium.

A cement known under the name of Araldite is a condensation product 2,2'-diphenylolpropane and epichlorohydrin.

Compositions of suitable ceramic materials for the sleeve are e. g.

76.2% powdered steatite 12.6% clay 7.9% barium carbonate 3.3% magnesite sintered at 1320 C., and

47.9% kaolin 15.9% clay 18.3% feldspar 17.9% quartz sintered at 1320 C.

The permanent magnets in accordance with the invention may, for example, be used in electric motors or electric generators in which permanent magnets rotate. The magnets in accordance with the invention may also be used for magnetic couplings.

What is claimed is:

1. In combination, a hollow cylindrical permanent magnet defining a central bore, said magnet consisting essentially of a composition MFerzOrs, M being a metal selected from the group consisting of barium, strontium, and lead, said magnet having a high retentivity and coercivity, a shaft centrally disposed within said bore and extending substantially parallel to the central axis of said magnet, and a cylindrical sleeve of a dense non-porous ceramic material disposed between and secured to the shaft and the magnet by cement.

2. The combination as claimed in claim 1 in which the diameter of the bore provided in the magnet is apthe cement consists of a synthetic resin. proximately 2 times the diameter of the shaft.

3. The combination as claimed in claim 1 in which the difference in diameter between the magnet and the References Cited 111 the file of thls patent sleeve and between the sleeve and the shaft is approxi- 5 UNITED STATES PATENTS infeterlgsgf min, the cement being constltuted by ethoxy- 2,475,321 Horn July 5, 1949 4. The combination as claimed in claim 1 in which 2543551 Krug 1951 

1. IN COMBINATION, A HOLLOW CYLINDRICAL PERMANENT MAGNET DEFINING A CENTRAL BORE, SAID MAGNET CONSISTING ESSENTIALLY OF A COMPOSITION MFE12O19, M BEING A METAL SELECTED FROM THE GROUP CONSISTING OF BARIUM, STRONTIUM, AND LEAD, SAID MAGNET HAVING A HIGH RETENTIVITY AND COERCIVITY, A SHAFT CENTRALLY DISPOSED WITHIN SAID BORE AND EXTENDING SUBSTANTIALLY PARALLEL TO THE CENTRAL AXIS OF SAID MAGNET, AND A CYLINDRICAL SLEEVE OF A DENSE NON-POROUS 